The present invention relates to a coextrusion multi-layer tubular film. More particularly, it is concerned with a coextrusion multi-layer tubular film which is superior in mechanical strength and transparency, and further which is free from curling even in the high-speed quenching molding method and does not cause blocking.
Heretofore, tubular films produced by inflation molding of low density polyethylenes produced by the high-pressure polymerization method have been widely used as packaging films for various purposes. These films, however, have a disadvantage in that the mechanical strength is not sufficiently high, although they are superior in transparency.
Compared with the above low density polyethylene films, those films made of linear low density ethylene .alpha.-olefin copolymers are superior in mechanical strength. In recent years, therefore, such linear low density ethylene .alpha.-olefin copolymer films have received increasing attention from a viewpoint of saving resources through a reduction in film thickness, and they are expected to be utilized in various fields. When, however, such linear low density ethylene .alpha.-olefin copolymers are molded by the commonly used inflation molding method, i.e., up blow air-cooling inflation molding, the ultimate tubular films are not always satisfactory in optical properties such as transparency and gloss. Moreover, in this air cooling method, the quenching effect is low, and if the air speed is increased, a fluctuation in resinous bubble occurs vigorously and the molding cannot be performed stably. In any case, films having superior transparency cannot be molded at high speeds and in a stabilized manner.
It has, therefore, been proposed to quench the resinous bubble by the down blow water-cooling inflation molding method which is known to have a high quenching effect. In accordance with this method, films can be produced which are greatly improved in transparency. On the contrary, the films are reduced in antiblocking properties. Thus they suffer from disadvantages that opening properties are poor and molding itself becomes difficult owing to the occurrence of blocking. This blocking may be prevented by increasing the amount of antiblocking agents added. If, however, the amount of antiblocking agents added is increased, they inevitably exert an adverse influence on transparency.
In the down blow water-cooling inflation molding method, the transparency of films can be further improved by increasing the degree of quenching. In this case, however, a fundamental and serious problem arises in that the film is curled toward the inside thereof. If this curling occurs, there can be obtained only films which have a appearance. Thus the films are low in product value. Moreover they have disadvantages in that handling during the molding and cutting process is difficult, workability for printing and production of bags drops, and the ultimate bags are bad in appearance due to the formation of stains, for example, in the production thereof. In the down blow water-cooling inflation molding method, although an improvement in transparency of films can be attained, the above problems of blocking and curling are undesirably involved.
As described above, tubular films of linear low density ethylene .alpha.-olefin copolymers, which are superior in transparency, have good opening properties and anti-blocking properties, and further are reduced in curling, have not yet been produced.